Single-sided distributed storage system

ABSTRACT

A distributed storage system including memory hosts and at least one curator in communication with the memory hosts. Each memory host has memory, and the curator manages striping of data across the memory hosts. In response to a memory access request by a client in communication with the memory hosts and the curator, the curator provides the client a file descriptor mapping data stripes and data stripe replications of a file on the memory hosts for remote direct memory access of the file on the memory hosts.

CROSS REFERENCE TO RELATED APPLICATIONS

This U.S. patent application is a continuation of, and claims priorityunder 35 U.S.C. § 120 from, U.S. patent application Ser. No. 16/508,578,filed on Jul. 11, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/885,918, filed on Feb. 1, 2018, which is acontinuation of U.S. patent application Ser. No. 14/987,443, filed onJan. 4, 2016, which is a continuation of U.S. patent application Ser.No. 13/492,346, filed on Jun. 8, 2012. The disclosures of these priorapplications are considered part of the disclosure of this applicationand are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This disclosure relates to distributed storage systems.

BACKGROUND

A distributed system generally includes many loosely coupled computers,each of which typically include a computing resource (e.g.,processor(s)) and storage resources (e.g., memory, flash memory, and/ordisks) A distributed storage system overlays a storage abstraction(e.g., key/value store or file system) on the storage resources of adistributed system. In the distributed storage system, a server processrunning on one computer can export that computer's storage resources toclient processes running on other computers. Remote procedure calls(RPC) may transfer data from server processes to client processes.

A remote procedure call is a two-sided software operation initiated byclient software executing on a first machine and serviced by serversoftware executing on a second machine. Servicing storage systemrequests (e.g., read data) in software may require an availableprocessor, which may place a significant limitation on a distributedstorage system. In the case of a distributed storage system, this meansa client process cannot access a remote computer's storage resourcesunless the remote computer has an available processor to service theclient's request. Moreover, the demand for processor resources andstorage resources in a distributed system often do not match. Inparticular, computing resource (i.e., processors) may have heavy and/orunpredictable usage patterns, while storage resources may have light andvery predictable usage patterns. When a server's processor(s) areheavily utilized, there may be no processors available to service astorage request when it arrives. In this case, the storage request waitsfor completion of other software tasks executed by the processor(s) orpreemption by a kernel before the storage request can be serviced by aprocessor, even though there may be plenty of storage bandwidth toservice the request immediately. Alternatively, one or more dedicatedprocessors could service such storage requests, at the expense ofefficiency and decreased processing capacity for nominal processingtasks.

Generally, coupling processor and storage resources can result in highand/or unpredictable latency, especially if the distributed system'sprocessor resources are heavily utilized.

SUMMARY

One aspect of the disclosure provides a distributed storage system thatincludes memory hosts and at least one curator in communication with thememory hosts. Each memory host has memory, and the curator managesstriping of data across the memory hosts. In response to a memory accessrequest by a client in communication with the memory hosts and thecurator, the curator provides the client a file descriptor mapping datastripes and data stripe replications of a file on the memory hosts forremote direct memory access of the file on the memory hosts.

Implementations of the disclosure may include one or more of thefollowing features. In some implementations, the distributed storagesystem includes a network, such as InfiniBand or Ethernet network,providing communication between the memory hosts and the client(s). Eachmemory host includes a network interface controller in communicationwith its memory (e.g., dynamic random access memory and/or phase changememory). The network interface controller services remote direct memoryaccess requests.

The curator may return location information of data on the memory hostsin response to the client memory access request In some examples, thecurator returns a key to allow access to data on the memory hosts inresponse to the client memory access request. The curator may allocatestorage of a data stripe on the memory hosts. Each file stored on thememory hosts may be divided into data stripes and each data stripe maybe replicated into multiple storage locations of the memory hosts. Insome implementations, the curator stores a file map mapping files tofile descriptors.

The file descriptor may include at least one of the following: a filestate attribute providing a state of a file, a data chunks attributeproviding a number of stripe replicas per stripe, a stripe lengthattribute providing a number of bytes per stripe, and a sub-stripelength attribute providing a number of bytes per sub-stripe. In someexamples, the file descriptor includes an array of stripe protocolbuffers, each describing a data stripe replica within a data stripe.Each stripe protocol buffer may include at least one of a stripe replicahandle, an identity of the memory host holding the stripe replica, and acurrent state of the stripe replica.

In some implementations, the distributed storage system includes cells.Each cell includes a curator and memory hosts in communication with thecurator. A transaction, in response to a client memory access request,executes at least one of a read operation and a write operation on filesstored in the memory hosts of that cell.

Another aspect of the disclosure provides a method of providing accessto data stored on a distributed storage system. The method includeselectronically receiving a memory access request from a client andreturning a file descriptor mapping data stripes and data stripereplications of a file on memory hosts for remote direct memory accessof the file on the memory hosts.

In some implementations, the method includes accessing a file mapmapping files to file descriptors to return the file descriptor inresponse to the memory access request. The method may include returninglocation information of data on the memory hosts in response to theclient memory access request. The method may include returning a key toallow access to data on the memory hosts in response to the clientmemory access request. In some examples, the method includes allocatingstorage of a data stripe on the memory hosts. The method may includedividing the file into data stripes and replicating each data stripeinto multiple storage locations of the memory hosts.

In some implementations, the method includes providing at least one of afile state attribute providing a state of a file, a data chunksattribute providing a number of stripe replicas per stripe, a stripelength attribute providing a number of bytes per stripe, and asub-stripe length attribute providing a number of bytes per sub-stripein the file descriptor. The method may include providing in the filedescriptor an array of stripe protocol buffers, each describing a datastripe replica within a data stripe.

Yet another aspect of the disclosure provides a computer program productencoded on a non-transitory computer readable storage medium comprisinginstructions that when executed by a data processing apparatus cause thedata processing apparatus to perform operations. The operations includeelectronically receiving a memory access request from a client andreturning a file descriptor mapping data stripes and data stripereplications of a file on memory hosts for remote direct memory accessof the file on the memory hosts.

In some implementations, the operations include accessing a file mapmapping files to file descriptors to return the file descriptor inresponse to the memory access request. The operations may includereturning location information of data on the memory hosts in responseto the client memory access request. The operations may includereturning a key to allow access to data on the memory hosts in responseto the client memory access request. In some examples, the operationsinclude allocating storage of a data stripe on the memory hosts. Theoperations may include dividing the file into data stripes andreplicating each data stripe into multiple storage locations of thememory hosts.

In some implementations, the operations include providing at least oneof a file state attribute providing a state of a file, a data chunksattribute providing a number of stripe replicas per stripe, a stripelength attribute providing a number of bytes per stripe, and asub-stripe length attribute providing a number of bytes per sub-stripein the file descriptor. The operations may include providing in the filedescriptor an array of stripe protocol buffers, each describing a datastripe replica within a data stripe.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description below. Otheraspects, features, and advantages will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an exemplary distributed storage system.

FIG. 2A is a schematic view of an exemplary distributed storage systemhaving a cell of memory hosts managed by a curator.

FIG. 2B is a schematic view of an exemplary cell of a distributedstorage system.

FIG. 3A is a schematic view of an exemplary file split into replicatedstripes.

FIG. 3B is a schematic view of an exemplary file descriptor.

FIG. 4 is a schematic view of an exemplary application programminginterface.

FIG. 5 is a schematic view of an exemplary curator.

FIG. 6 provides an exemplary arrangement of operations for a method ofproviding access to data stored on a distributed storage system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIGS. 1-3, in some implementations, a distributed storagesystem 100 includes loosely coupled memory hosts 110, 110 a-n (e.g.,computers or servers), each having a computing resource 112 (e.g., oneor more processors) in communication with storage resources 114 (e.g.,memory, flash memory, dynamic random access memory (DRAM), phase changememory (PCM), and/or disks). A storage abstraction (e.g., key/valuestore or file system) overlain on the storage resources 114 allowsscalable use of the storage resources 114 by one or more clients 120,120 a-n. The clients 120 may communicate with the memory hosts 110through a network 130. Rather than having a processor 112 of a memoryhost 110 (e.g., a server) execute a server process that exports accessof the corresponding storage resource 114 (e.g., memory) to clientprocesses executing on the clients 120, the clients 120 may directlyaccess the storage resource 114 through a network interface controller(NIC) 116 of the memory host 110. In other words, a client processexecuting on a client 120 may directly interface with one or morestorage resources 114 without requiring execution of a routine of anyserver processes executing on the computing resources 112. This offers asingle-sided distributed storage architecture that offers relativelyhigh-throughput and low latency, since clients 120 can access thestorage resources 114 without interfacing with the computing resources112 of the memory hosts 110.

A network interface controller 116 (also known as a network interfacecard, network adapter, or LAN adapter) may be a computer hardwarecomponent that connects a computing resource 112 to the network 130. Thenetwork controller 116 implements communication circuitry using aspecific physical layer (OSI layer 1) and data link layer (layer 2)standard, such as Ethernet, Wi-Fi, or Token Ring. This provides a basefor a full network protocol stack, allowing communication among smallgroups of computers on the same LAN and large-scale networkcommunications through routable protocols, such as Internet Protocol(IP).

In some implementations, the network 130 is an InfiniBand network, whichis a switched fabric communications link generally used inhigh-performance computing and enterprise data centers. It features highthroughput, low latency, quality of service, failover, and scalability.The InfiniBand architecture specification defines a connection betweenprocessor nodes and high performance I/O nodes such as storage devices.The InfiniBand network 130 conveys remote direct memory access (RDMA)requests 122 from a client 120 to a memory host 110. At the memory host110, an RDMA-capable InfiniBand network interface controller (NIC) 116performs reads and writes of the storage resource 114 (e.g., DRAM). RDMAuses zero-copy, OS-bypass to provide high throughput, low latency accessto data (e.g., 4 GB/s of bandwidth and 5 microsecond latency). Thedistributed storage system 100 may use RDMA, remote procedure calls, orother data access methods to access data.

Referring to FIGS. 2A and 2B, in some implementations, the distributedstorage system 100 includes multiple cells 200, each cell 200 includingmemory hosts 110 and a curator 210 in communication with the memoryhosts 110. Each cell 200 may also include one or more stripe doctors 220(e.g., processes for managing and/or repairing stripes), one or moreslowpoke clients 230 (e.g., clients or virtual clients used forassessing system performance), and a console 240 for monitoring andmanaging the cell 200.

The curator 210 (e.g., process) may execute on a computing processor 202(e.g., server) connected to the network 130 and manages the data storage(e.g., manages a file system stored on the memory hosts 110), controlsdata placements, and/or initiates data recovery. Moreover, the curator210 may track an existence and storage location of data on the memoryhosts 110. Redundant curators 210 are possible. In some implementations,the curator(s) 210 track the striping of data across multiple memoryhosts 110 and the existence and/or location of multiple copies of agiven stripe for redundancy and/or performance. In computer datastorage, data striping is the technique of segmenting logicallysequential data, such as a file, in a way that accesses of sequentialsegments are made to different physical storage devices (e.g., cells 200and/or memory hosts 110). Striping is useful when a processing devicerequests access to data more quickly than a storage device can provideaccess. By performing segment accesses on multiple devices, multiplesegments can be accessed concurrently. This provides more data accessthroughput, which avoids causing the processor to idly wait for dataaccesses.

In some implementations, an application programming interface (API) 400interfaces between a client 120 and the curator 210. In some examples,the client 120 communicates with the curator 210 through one or moreremote procedure calls (RPC). In response to a client request 122, theAPI 400 may find the storage location of certain data on memory host(s)110, and obtain a key that allows access to the data. The API 400communicates directly with the appropriate memory hosts 110 to read orwrite the data (e.g., using remote direct memory access). In the casethat a memory host 110 is non-operational, or the data was moved to adifferent memory host 110, the client request 122 fails, prompting theclient 120 to re-query the curator 210.

Referring to FIGS. 3A and 3B, in some implementations, the memory hosts110 store file data. The curator 210 may divide each file 310 intostripes 320 a-n and replicate the stripes 320 a-n in multiple storagelocations. A stripe replica 320 n _(k) is also referred to as a chunk.Mutable files have additional metadata stored on the memory host(s) 110,such as lock words and version numbers. The lock words and versionsnumbers may be used to implement a distributed transaction commitprotocol.

File descriptors 300 stored by the curator 210 contain metadata thatmaps the stripes 320 a-n to chunks 320 n _(k) (i.e., stripe replicas) onthe memory hosts 110. To open a file 310, a client 120 sends a request122 to the curator 210, which returns a file descriptor 300. The client120 uses the file descriptor 300 to translate file chunk offsets toremote memory locations. After the client 120 loads the file descriptor300, the client 120 may access the file's data via RDMA or another dataretrieval method.

In some implementations, the distributed storage system 100 supports twotypes of files: immutable and mutable. Immutable files rely on adisk-based file system for persistence and fault-tolerance. A client 120may copy immutable files into the file system of the distributed storagesystem 100. On the other hand, a client 120 may write mutable files intothe file system of the distributed storage system 100 using theapplication programming interface (API) 400. The storage system 100 mayor may not be durable. The distributed storage system 100 may havestrict data loss service level objectives (SLOs) that depend on thefiles' level of replication. When a stripe 320 n is lost, the curator210 may allocate new storage for the lost stripe 320 n and mark the dataas uninitialized. A client 120 attempting to read an uninitializedstripe 320 n receives an uninitialized data error. At this point, theclient 120 can reinitialize the stripe's data.

The file descriptor 300 may provide the state of a file 310. A file canbe in one of the following states: READ, READ_WRITE, DELETED, or{CREATE, COPY, RESIZE}_PENDNG. In the READ state, clients 120 can readthe file 310, but not write to the file 310. Read-only files 310 areread-only for the entire life-time of the file 310, i.e., read-onlyfiles 310 are never written to directly. Instead, read-only files 310can be copied into the file system from another file system. A backingfile 310 may be used to restore data when a memory host 110 crashes;consequently, the backing file 310 persists for the entire life-time ofthe file 310. In the READ_WRITE state, clients 120 with the appropriatepermissions can read and write a mutable file's contents. Mutable files310 support concurrent, fine grain, random writes. Random and sequentialwrite performance may be comparable. Writes are strongly consistent,that is, if any client 120 can observe the effect of a write, then allclients 120 can observe the effect of a write. Writes can also bebatched into transactions. For example, a client 120 can issue a batchof asynchronous writes followed by a sync operation. Strong consistencyand transactional semantics ensure that if any client 120 can observeany write in a transaction, then all clients 120 can observe all writesin a transaction. In the DELETED state, the file 310 has been deleted.The chunks 320 n _(k) belonging to the file 310 are stored in a deletedchunks field and wait for garbage collection. The {CREATE, COPY,RESIZE}_PENDING state denotes a file 310 has a create, copy, or resizeoperation pending on the file.

An encoding specified by a file encoding protocol buffer of the filedescriptor 300 may be used for all the stripes 320 a-n within a file310. In some examples, the file encoding contains the following fields:“data chunks.” which provides a number of data chunks 320 n _(k) perstripe 320 n; “stripe length,” which provides a number of bytes perstripe 320 n; and “sub-stripe length,” which provides a number of bytesper sub-stripe. The sub-stripe length may be only valid for READ_WRITEfiles. The data for a file 310 may be described by an array of stripeprotocol buffers 325 in the file descriptor 300. Each stripe 320 nrepresents a fixed region of the file's data, identified by an indexwithin the array. The contents of a stripe 320 n may include an array ofchunk protocol buffers, each describing a chunk 320 n _(k) within thestripe 320 n, including a chunk handle, an identity of the memory host110 holding the chunk 320 n _(k), and a current state of the chunk 320 n_(k). For RDMA purposes, the chunk protocol buffers may also store avirtual address of the chunk 320 n _(k) in the memory host 110 and a32-bit r-key. The r-key is unique to a chunk 320 n _(k) on a memory host110 and is used to RDMA-read that chunk 320 n _(k).

Chunks 320 n _(k) can be in one of the following states: OK, Recovering,Migrating Source, and Migrating Destination. In the OK state, thecontents are valid and the chunk 320 n _(k) contributes to thereplication state of the corresponding stripe 320 n. Clients 120 mayupdate all chunks 320 n _(k) in a good state. In the Recovering state,the chunk Recovering is in the process of being recovered. The chunkRecovering does not count towards the replicated state of thecorresponding stripe 320 n and the data in the chunk 320 n is notnecessarily valid. Therefore, clients 120 cannot read data from chunks320 n _(k) in the Recovering state. However, all transactions notreaching their commit point at the time a chunk state changes to theRecovering state must include the Recovering chunk in the transaction inorder to ensure that the chunk's data is kept up to date duringrecovery. In the Migrating Source state, the chunk 320 n _(k) is in theprocess of migrating. A migrating source attribute may provide alocation from which the chunk 320 n _(k) is migrating. The source chunk320 n _(k) counts towards the replication of the stripe 320 n and thedata in the chunk 320 n _(k) is valid and can be read. In the MigratingDestination state, the chunk is in the process of migrating. A MigratingDestination attribute provides the location to which the chunk 320 n_(k) is migrating. The source chunk 320 n _(k) does not count towardsthe replicated state of the stripe 320 n and the chunk 320 n _(k) is notnecessarily valid. Therefore, clients 120 cannot read from chunks 320 n_(k) in the Migrating Destination state. However, all transactions notreaching their commit point at the time a chunk's state changes to theMigrating Destination state must include the Migrating Destination chunk320 n _(k) in the transaction in order to ensure the chunk's data iskept up to date as it is being migrated.

Each file descriptor 300 may have a dead chunks array. The dead chunksarray holds additional chunks 320 n _(k) that are no longer needed, suchas the chunks 320 n _(k) that made up a file 310 that has since beendeleted, or made up previous instances of the file 310. When the file310 is deleted or truncated, the chunks 320 n _(k) from all the stripes320 n are moved into this dead chunks array and the stripes 320 n arecleared. The chunks 320 n _(k) in the dead chunks array are reclaimed inthe background.

The application programming interface 400 may facilitate transactionshaving atomicity, consistency, isolation, durability (to a degree), suchthat the transaction may be serializable with respect to othertransactions ACID (atomicity, consistency, isolation, durability) is aset of properties that guarantee that database transactions areprocessed reliably In the context of databases, a single logicaloperation on the data is called a transaction. Atomicity requires thateach transaction is “all or nothing”: if one part of the transactionfails, the entire transaction fails, and the database state is leftunchanged. An atomic system guarantees atomicity in each and everysituation, including power failures, errors, and crashes. Consistencyensures that any transaction brings the database from one valid state toanother. Any data written to the database must be valid according to alldefined rules, including but not limited to constraints, cascades,triggers, and any combination thereof. Isolation ensures that notransaction should be able to interfere with another transaction. Oneway of achieving this is to ensure that no transactions that affect thesame rows can run concurrently, since their sequence, and hence theoutcome, might be unpredictable. This property of ACID may be partlyrelaxed due to the huge speed decrease this type of concurrencymanagement entails. Durability means that once a transaction has beencommitted, it will remain so, even in the event of power loss, crashes,or errors. In a relational database, for instance, once a group of SQL,statements execute, the results need to be stored permanently. If thedatabase crashes immediately thereafter, it should be possible torestore the database to the state after the last transaction committed.

Referring to FIG. 4, in some implementations, the applicationprogramming interface (API) 400 includes a reader class 410 and atransaction class 420. A client 120 may instantiate a reader 410 ainheriting the reader class 410 to execute a read or batches of reads onthe memory hosts 110 in a cell 200. Moreover, the client 120 mayinstantiate a transaction 420 a inheriting the transaction class 420 toexecute one or more reads and/or writes. The reads and writes in atransaction 420 a may be to different files 310 in a cell 200, but insome implementations, all reads and writes in a transaction must be tofiles 310 in the same cell 200. Reads may be executed and are “snapshotconsistent,” meaning that all reads in a transaction 420 a can see asnapshot of the file 310 at a logical instant in time. Writes can bebuffered until the client 120 tries to commit the transaction 420 a.

When a client 120 adds a file read request 122 r to the reader 410 a,the reader 410 a translates the read request 122 r into a RDMA readnetwork operation and stores a state of the network operation in memoryallocated for the reader 410 a. Reads that cross chunk boundaries gettranslated into multiple RDMA operations.

In some implementations, to translate a file read request 122 r into aRDMA read network operation, the reader 410 a computes a target stripenumber from a file offset of the read request 122 r. The reader 410 amay use the stripe number to index into a chunk handle cache. The chunkhandle cache returns a network channel to access the corresponding chunk320 n _(k) and a virtual address and r-key of the chunk 320 n _(k) Thereader 410 a stores the network channel and r-key directly in anoperation state of the RDMA read. The reader 410 a uses the virtualaddress of the chunk 320 n _(k) and the file offset to compute thevirtual address within the chunk 320 n _(k) to read. The reader 410 acomputes the offset into a memory block supplied by the client 120(e.g., a receiving memory block for each RDMA read operation). Thereader 410 a may then initialize an operation status.

While buffering new reads, the reader 410 may calculate and store arunning sum of the amount of metadata that will be retrieved to completethe read. This allows metadata buffer space to be allocated in onecontiguous block during execution, minimizing allocation overhead.

In some implementations, the reader 410 a executes a read operation intwo phases. In the first phase, the reader 410 a reads the data andassociated metadata of a file 310. In the second phase, the reader 410 avalidates that the data read in the first phase satisfies dataconsistency constraints of the reader 410 a. In the first phase, thereader 410 a transmits its RDMA read operations. While iterating throughand transmitting RDMA reads, the reader 410 a initializes and transmitsRDMA reads to read sub-chunk metadata and to read data needed to computechecksums of the first and last sub-chunks in an unaligned file access.After the data and metadata are received, the reader 410 a may checklock-words in the sub-chunk metadata to ensure that the sub-chunks werenot locked while the data was being read. If a sub-chunk was locked, thereader 410 a rereads the sub-chunk and its corresponding metadata. Oncethe reader 410 a finds (reads) all of the sub-chunk locks in an unlockedstate, the reader 410 a computes the sub-chunk checksums and comparesthe computed checksums with the checksums read from the sub-chunkmetadata.

A sub-chunk checksum may fail a compare for one of three reasons 1) thedata read was corrupted by a concurrent write; 2) the data was corruptedwhile in transit to the client; or 3) the data stored in the memory hostis corrupt. Cases 1 and 2 are transient errors. Transient errors areresolved by retrying the sub-chunk read. Case 3 is a permanent errorthat may require the client to notify the curator of a corruptsub-stripe 322 n.

To differentiate between a transient error and a permanent error, theclient 120 may re-read the sub-chunk data and metadata. The reader 410 athen checks the sub-chunk lock-word and re-computes and compares thesub-chunk checksum. If the checksum error still exists and the sub-chunkversion number has changed since the sub-chunk was initially read, thenthe checksum compare failure was likely caused by a concurrent write sothe reader 410 a retries the sub-chunk read. If the version number hasnot changed since the sub-chunk was initially read, then the error ispermanent and the reader 410 a notifies the curator 210, and the curator210 tries to reconstruct the data of the chunk 320 n _(k). If thecurator 210 is unable to reconstruct the chunk data, the curator 210replaces the old chunk 320 n _(k) with a new uninitialized chunk 320 n_(k).

Sub-chunk locks may become stuck due to a client 120 trying to execute atransaction 420 a but crashing during a commit protocol of thetransaction 420 a. A reader 410 a can detect a stuck lock by re-readingthe sub-chunk lock-word and version number. If a sub-chunk lock-word andversion number do not change during some time out period, then thesub-chunk lock is likely stuck. When the reader 410 a detects a stucklock, it notifies the curator 210 of the stuck lock and the curator 210recovers the sub-stripe 322 n and resets the stuck lock.

After the reader 410 a validates each sub-chunk lock-word and checksum,the reader 410 a may proceed to the second phase of executing the readoperation (i.e., the validation phase). To validate the values, thereader 410 a rereads sub-chunk metadata and rechecks if the sub-chunklock-words are unlocked and the sub-chunk version numbers have notchanged since the version numbers were initially read during the firstphase of the read operation. If the reader 410 a is associated with atransaction 420 a, the reader 410 a may reread the metadata associatedwith all sub-chunks read by the transaction 420 a. If a single sub-chunkversion number mis-compares, the reader 410 a returns an error. If allsub-chunk version numbers are the same, the reader 410 a discards theprefix and suffix of the reader memory block in order to trim extraneousdata read to compute the checksum of the first and last sub-chunks inthe read. The reader 410 a may set a status to OK and returns to theclient 120.

If the reader 410 a encounters an error on a network channel whilereading data or metadata of a chunk, the reader 410 a may select adifferent chunk 320 n _(k) from the chunk handle cache and notifies thecurator 210 of a bad memory host. If no other good chunks 3201 n _(k)exist from which the reader 410 a can read, the reader 410 a may wait toreceive a response to the error notification it sent to the curator 210.The response from the curator 210 may contain an updated file descriptor300 that contains a new good chunk 320 n _(k) to read from.

In some implementations, the transaction class 420 uses validation sets422 to track which sub-stripes 322 n have been read by the transaction420 a. Each read of a transaction 420 a adds the version numbers of allsub-stripes 322 n read to a validation set 422 of the transaction 420 a.The transaction 420 a may validate the validation set 422 in twocases: 1) as part of the commit protocol and 2) the validation phase ofreads of a transaction 420 a. A transaction 420 a may fail to commit ifthe commit protocol finds that any sub-stripe version number differsfrom the number recorded in the validation set 422. Validation of thefull validation set 422 before data is returned to the client 120 allowsearly detection (e.g., before the commit phase) of a doomed transaction420 a. This validation also prevents the client 120 from getting aninconsistent view of file data.

A transaction 420 a may provide a synchronous, serializable readoperation (e.g., using a reader). In some examples, a reader 410 a isinstantiated and associated with the transaction 420 a. Read results ofthe reader 410 a return the latest committed data. As such, uncommittedwrites of the same transaction 420 a are not seen by a read of thattransaction 420 a.

A transaction 420 a may buffer data for a later transaction commit. Thetransaction class 420 translates a buffer write request into one or more‘prepare write’ network operations. One network operation is needed foreach stripe 320 n touched by the write operation. Processing a bufferwrite request may involve preparing ‘sub-stripe lock’ networkoperations. One lock operation is needed for each sub-stripe 322 ntouched by the requested write. These operations are buffered fortransmission during the transaction commit. The transaction 420 a maytranslate buffer write requests into network operations and executeidentify or coalesce writes that affect the same region of a file 310.The transaction 420 a may apply write operations in the same order bythe memory hosts 110 for all chunks 320 n _(k) to ensure that allreplicas are consistent.

The transaction 420 a may provide a commit operation that results in allreads and writes in the transaction 420 a being schedulable as a singleatomic, serializable operation In some implementations, the transactioncommit protocol proceeds through a lock phase, a validate phase, a writephase, and an unlock phase. During the lock phase, the sub-stripe locknetwork operations which were created in response to buffer writerequests are sent. Each sub-stripe lock operation executes an atomiccompare-and-swap operation on the lock-word in all replicas 320 n _(k).If the client 120 succeeds in writing its unique client ID into themetadata lock-word, it has successfully taken the lock. If thetransaction 420 a fails to take the lock for any sub-stripe 322 n in thewrite set, the commit fails and is aborted. The commit protocol proceedsto the validate phase once all sub-stripe locks are held.

During the validate phase, the transaction 420 a may read the versionnumber out of the metadata for all sub-stripes 322 n referenced in thevalidation set and comparing the version numbers to the version numbersrecorded in the validation set. If a version number does not match, thesub-stripe 322 n was written by another transaction after it was read bythis transaction, so the transaction must fail. In this case, the reader410 a releases the locks it holds and returns a transaction conflicterror to the client 120. Once all version numbers in the validation sethave been validated, the client 120 writes the buffered write data ofthe transaction 420 a to each replica 320 n and updates the metadataassociated with each sub-stripe 322 n written by the transaction 420 a,during the write phase. Updating metadata of a sub-stripe 322 n mayinclude computing and writing a new check-word and incrementing theversion number of the sub-stripe 322 n. Once all data and metadata hasbeen updated, the transaction 420 a releases the locks that it holds,during the unlock phase.

Referring to FIG. 5, in some implementations, the curator 210 stores andmanages file system metadata 500. The metadata 500 includes a file map510 that maps files 310 _(1-n) to file descriptors 300 _(1-n). Thecurator 210 may examine and modify the representation of its persistentmetadata 500. The curator 210 may use three different access patternsfor the metadata 500: read-only, file transactions, and stripetransactions. Read-only access allows the curator 210 to examine a stateof the metadata 500 with minimal contention. A read-only request returnsthe most recent state of a file 310, but with no synchronization withconcurrent updates. The read-only access may be used to respond tolookup requests from clients 120 (e.g., for internal operations, such asfile scanning).

File transaction access may provide exclusive read/write access to thestate of a file descriptor 300. Updates to the file state may be appliedat the end of a transaction 420 a and are atomic. File transactionaccess can be used for operations such as creating, finalizing, anddeleting a file 310. These operations may require the curator 210 tocommunicate with other components such as memory hosts and thus a filetransaction access may last for several seconds or more. While active,the file transaction access blocks any other operations that need tomodify the state of the file descriptor 300. Read access may not beblocked.

To reduce contention, stripe transaction access may provide relativelyfiner grain synchronization for operations that only need to modify thestate of a single stripe 320 n with the file descriptor 300. This modecan be used for stripe operations such as opening, closing, rebalancing,and recovering. There can be many concurrent stripe transactions fordifferent stripes 320 n within a file 310, but stripe transactions andfile transactions are mutually exclusive. Within a stripe transaction,the curator 210 may examine the state of a stripe 320 n and variousfields of the file descriptor 300 that remain immutable for the durationof the transaction 420 a, such as the file encoding and instanceidentifier. The stripe transaction access does not provide access tofields that can change underfoot, such as the state of other stripes 320n. Operations may hold only one active transaction 420 a at a time toavoid deadlock. Moreover, transactions 420 a may only atomically commiton a single file 310.

In some implementations, the curator 210 can create, copy, resize, anddelete files. Other operations are possible as well. To service a copyrequest 122 c from a client 120, the curator 210 creates a new filedescriptor 300 having a state initially set to COPY_PENDING. The curator210 may set/initialize one or more of the following fields: size, owner,group, permissions, and/or backing file. The curator 210 populates astripes array 325 of the file descriptor 300 (FIG. 3B) with emptystripes 320 n and then commits the file descriptor 300 to its file map510. Committing this information to the file map 510 allows the curator210 to restart a resize operation if the curator 210 crashes or a tabletcontaining the file system metadata 500 migrates to another curator 210.Once the curator 210 commits file descriptor 300 to the file map 510,the curator 210 responds to the client copy request 122 c by informingthe client 120 that the copy operation has been initiated. The curator210 initiates memory-host-pull-chunk operations, which instruct memoryhosts 110 to allocate a new chunk 320 n _(k) and to read chunks 320 n_(k) of the backing file into the memory 114 of the memory hosts 110.When a pull-chunk operation returns successfully, the curator 210 addsthe new chunk 320 n _(k) to the appropriate stripe 320 n in the filedescriptor 300. The curator 210 commits the stripe 320 n with the newchunk 320 n _(k) to the file map 510.

In the case of a crash or a migration, incrementally updating the filedescriptors 300 allows a new curator 210 to restart a copy operationfrom the location the prior curator 210 stopped. This also allowsclients 120 to check the status of a copy operation by retrieving thefile descriptor 300 (e.g., via a lookup method) and inspecting thenumber of stripes 320 n in the file descriptor 300 populated with chunks320 n _(k). Once all chunks 320 n _(k) have been copied to the memoryhosts 110, the curator 210 transitions the state of the file descriptor300 to READ and commits it to the file map 510.

The curator 210 may maintain status information for all memory hosts 110that are part of the cell 200. The status information may includecapacity, free space, load on the memory host 110, latency of the memoryhost 110 from a client's point of view, and a current state. The curator210 may obtain this information by querying the memory hosts 110 in thecell 200 directly and/or by querying slowpoke clients 230 to gatherlatency statistics from a client's point of view. In some examples, thecurator 210 uses the memory host status information to make rebalancing,draining, recovery decisions, and allocation decisions.

The curator(s) 210 may allocate chunks in order to handle clientrequests 122 for more storage space in a file 310 and for rebalancingand recovery. The curator 210 may maintain a load map 520 of memory hostload and liveliness. In some implementations, the curator 210 allocatesa chunk 320 n _(k) by generating a list of candidate memory hosts 110and sends an allocate chunk requests to each of the candidate memoryhosts 110. If the memory host 110 is overloaded or has no availablespace, the memory host 110 can deny the request In this case, thecurator 210 selects a different memory host 110. Each curator 210 maycontinuously scan its designated portion of the file namespace,examining all the metadata 500 every minute or so. The curator 210 mayuse the file scan to check the integrity of the metadata 500, determinework that needs to be performed, and/or to generate statistics. The filescan may operate concurrently with other operations of the curator 210.The scan itself may not modify the metadata 500, but schedules work tobe done by other components of the system and computes statistics.

For each file descriptor 300, the file scan may: ensure that the filedescriptor 300 is well formed (e.g., where any problems may indicate abug in either the curator or in the underlying storage of the metadata);update various statistics, such as the number of files 310, stripes 320n, chunks 320 n _(k), and the amount of storage used; look for stripes320 n that need recovery, determine if the file descriptor 300 containschunks 320 n _(k) that are candidates for rebalancing from overfullmemory hosts 110; determine if there are chunks 320 n _(k) on drainingmemory hosts 110, determine if there are chunks 320 n _(k) that arecandidates for rebalancing to under-full memory hosts 110; determinechunks 320 n _(k) that can be deleted; and/or determine if the filedescriptor 300 has a pending resize or copy operation, but there is noactive task within the curator 210 working on the operation.

Referring again to FIGS. 2A and 2B, the distributed storage system 100may include one or more stripe doctors 220 in each cell 200 that fix andrecover stripe data. For example, each cell 200 may have several stripedoctors 220 that execute rebalancing and recovery operations.Additionally or alternatively, the distributed storage system 100 mayinclude one or more slowpoke clients 230 that monitor a cell'sperformance from a client's prospective. A slowpoke client 230 provideslatency information to the curator(s) for chunk allocation andrebalancing. For example, each slowpoke client 230 collects latencystatistics for every memory host 110 in a cell. A cell 200 may haveseveral slowpoke clients 230 to improve network path coverage and faulttolerance. Since it is unlikely that the curator may completely lose thelatest latency statistics for a memory host 110 in the cell 200,curators 210 can make chunk allocation and rebalancing decisions withoutthe slowpoke latency information.

FIG. 6 provides an exemplary arrangement 600 of operations for a methodof providing access to data stored on a distributed storage system. Themethod includes electronically receiving 602 a memory access request 122from a client 120 and returning a file descriptor 300 mapping datastripes 320 n and data stripe replications 320 n _(k) of a file 310 onmemory hosts 110 for remote direct memory access (RDMA) of the file 310on the memory hosts 110.

In some implementations, the method includes accessing 606 a file map510 mapping files 310 to file descriptors 300 to return the filedescriptor 300 in response to the memory access request 122. The methodmay include returning 608 location information of data on the memoryhosts 110 in response to the client memory access request 122. Themethod may include returning a key to allow access to data on the memoryhosts 110 in response to the client memory access request 122. In someexamples, the method includes allocating storage of a data stripe 320 non the memory hosts 110. The method may include dividing the file 310into data stripes 320 n and replicating each data stripe 320 n intomultiple storage locations of the memory hosts 110.

In some implementations, the method includes providing at least one of afile state attribute providing a state of a file, a data chunksattribute providing a number of stripe replicas 320 n _(k) per stipe 320n, a stripe length attribute providing a number of bytes per stripe 320n, and a sub-stripe length attribute providing a number of bytes persub-stripe 322 n in the file descriptor 300. The method may includeproviding 610 in the file descriptor 300 an array of stripe protocolbuffers 325, each describing a data stripe replica 320 n _(k) within adata stripe 320 n.

Servicing storage requests 122 in hardware provides a number ofadvantages, such as having relatively simple storage requests (e.g.,read, write). Implementing such functionality in an application specificintegrated circuit (ASIC) can be much more efficient than implementingthe functionality in software running on a general-purpose processor.This efficiency improvement means storage requests 122 can be servicedin less time and occupy fewer circuits when compared to implementing thesame functionality in software running on a general-purpose processor.In turn, this improvement means a distributed storage system 100 canachieve lower latency and higher throughput without increasing the costof the system.

Servicing storage requests 122 in the network interface hardware (e.g.,NIC) decouples processor resources 112 and storage resources 114. Aclient 120 can access storage resources 114 on a memory host 110 withoutavailable processor resources 112. This allows system builders andadministrators to operate a distributed storage system 100 with highprocessor utilization, with low and predictable latencies, and withoutstranding storage resources. In some implementations, the distributedstorage system 100 can provide an average read latency of less than 10microseconds, an average read throughput of greater than 2 millionoperations per second per client, and average write latency of less than50 microseconds, and/or an average write throughput of greater than 500thousand operations per second per client.

Various implementations of the systems and techniques described here canbe realized in digital electronic circuitry, integrated circuitry,specially designed ASICs (application specific integrated circuits),computer hardware, firmware, software, and/or combinations thereof.These various implementations can include implementation in one or morecomputer programs that are executable and/or interpretable on aprogrammable system including at least one programmable processor, whichmay be special or general purpose, coupled to receive data andinstructions from, and to transmit data and instructions to, a storagesystem, at least one input device, and at least one output device.

These computer programs (also known as programs, software, softwareapplications or code) include machine instructions for a programmableprocessor, and can be implemented in a high-level procedural and/orobject-oriented programming language, and/or in assembly/machinelanguage. As used herein, the terms “machine-readable medium” and“computer-readable medium” refer to any computer program product,apparatus and/or device (e.g., magnetic discs, optical disks, memory,Programmable Logic Devices (PLDs)) used to provide machine instructionsand/or data to a programmable processor, including a machine-readablemedium that receives machine instructions as a machine-readable signal.The term “machine-readable signal” refers to any signal used to providemachine instructions and/or data to a programmable processor.

Implementations of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. Moreover,subject matter described in this specification can be implemented as oneor more computer program products, i.e., one or more modules of computerprogram instructions encoded on a computer readable medium for executionby, or to control the operation of, data processing apparatus. Thecomputer readable medium can be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter effecting a machine-readable propagated signal, or a combinationof one or more of them. The terms “data processing apparatus”,“computing device” and “computing processor” encompass all apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them. A propagated signal is an artificially generated signal, e.g.,a machine-generated electrical, optical, or electromagnetic signal, thatis generated to encode information for transmission to suitable receiverapparatus.

A computer program (also known as an application, program, software,software application, script, or code) can be written in any form ofprogramming language, including compiled or interpreted languages, andit can be deployed in any form, including as a stand alone program or asa module, component, subroutine, or other unit suitable for use in acomputing environment. A computer program does not necessarilycorrespond to a file in a file system. A program can be stored in aportion of a file that holds other programs or data (e.g., one or morescripts stored in a markup language document), in a single filededicated to the program in question, or in multiple coordinated files(e.g., files that store one or more modules, sub programs, or portionsof code). A computer program can be deployed to be executed on onecomputer or on multiple computers that are located at one site ordistributed across multiple sites and interconnected by a communicationnetwork.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Moreover, a computer can be embedded inanother device, e.g., a mobile telephone, a personal digital assistant(PDA), a mobile audio player, a Global Positioning System (GPS)receiver, to name just a few. Computer readable media suitable forstoring computer program instructions and data include all forms of nonvolatile memory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices, magnetic disks, e.g., internal hard disks or removable disks,magneto optical disks; and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, one or more aspects of thedisclosure can be implemented on a computer having a display device,e.g., a CRT (cathode ray tube), LCD (liquid crystal display) monitor, ortouch screen for displaying information to the user and optionally akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

One or more aspects of the disclosure can be implemented in a computingsystem that includes a backend component, e.g., as a data server, orthat includes a middleware component, e.g., an application server, orthat includes a frontend component, e.g., a client computer having agraphical user interface or a Web browser through which a user caninteract with an implementation of the subject matter described in thisspecification, or any combination of one or more such backend,middleware, or frontend components. The components of the system can beinterconnected by any form or medium of digital data communication,e.g., a communication network. Examples of communication networksinclude a local area network (“LAN”) and a wide area network (“WAN”), aninter-network (e.g., the Internet), and peer-to-peer networks (e.g., adhoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someimplementations, a server transmits data (e.g., an HTML page) to aclient device (e.g., for purposes of displaying data to and receivinguser input from a user interacting with the client device). Datagenerated at the client device (e.g., a result of the user interaction)can be received from the client device at the server.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of the disclosure or of what maybe claimed, but rather as descriptions of features specific toparticular implementations of the disclosure. Certain features that aredescribed in this specification in the context of separateimplementations can also be implemented in combination in a singleimplementation. Conversely, various features that are described in thecontext of a single implementation can also be implemented in multipleimplementations separately or in any suitable sub-combination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multi-tasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. Accordingly, otherimplementations are within the scope of the following claims. Forexample, the actions recited in the claims can be performed in adifferent order and still achieve desirable results.

What is claimed is:
 1. A method comprising: receiving, at dataprocessing hardware, a copy request from a client, the copy requestrequesting replication of a file stored on memory hosts of a distributedstorage system, the file divided into data stripes stored acrossmultiple source storage locations of the memory hosts, each memory hosthaving memory; populating, by the data processing hardware, a data arrayto have a size corresponding to a number of the data stripes in thefile, the data array configured to receive data stripe replications ofthe data stripes in the file; instructing, by the data processinghardware, the memory hosts to: allocate storage of each of the datastripe replications across multiple destination storage locations of thememory hosts; and migrate each of the data stripe replications from thesource storage locations to the destination storage locations of thememory hosts; and committing, by the data processing hardware, each ofthe data stripe replications to the data array.
 2. The method of claim1, further comprising determining, by the data processing hardware, astate of each data stripe replication, the state indicating whether arespective data stripe replication is available for allocation to arespective destination storage location.
 3. The method of claim 1,further comprising receiving, at the data processing hardware, a memoryaccess request for the file; determining, by the data processinghardware, that one or more data stripe replications of the file are in amigrating state to a respective destination storage location of thememory hosts; and denying, by the data processing hardware, the memoryaccess request for the file.
 4. The method of claim 1, whereininstructing the memory hosts further comprises instructing the memoryhosts to read the data stripes stored across multiple source storagelocations of the distributed storage system.
 5. The method of claim 1,further comprising accessing, by the data processing hardware, a filemap mapping files to file descriptors to return a file descriptorcorresponding to the data array in response to the copy request, thefile descriptor configured to map the data stripe replications of thedata stripes on respective memory hosts of the distributed storagesystem.
 6. The method of claim 1, further comprising providing, by thedata processing hardware, at least one of a file state attributeproviding a state of the file, a data chunks attribute providing anumber of data stripe replications per stripe, a stripe length attributeproviding a number of bytes per stripe, or a sub-stripe length attributeproviding a number of bytes per sub-stripe.
 7. The method of claim 1,wherein the copy request is received across a network providingcommunication between the memory hosts and the client, each memory hostcomprising a network interface controller in communication with itsmemory.
 8. The method of claim 7, wherein the network interfacecontroller services remote direct memory access requests.
 9. The methodof claim 7, wherein the network comprises at least one of an InfiniBandnetwork or an Ethernet network.
 10. The method of claim 1, wherein thememory comprises at least one of dynamic random access memory and phasechange memory.
 11. A distributed storage system comprising: memoryhosts, each memory host having memory; and data processing hardware incommunication with the memory hosts, the data processing hardwaremanaging files stored across the memory hosts by: receiving a copyrequest from a client, the copy request requesting replication of a filestored on the memory hosts, the file divided into data stripes storedacross multiple source storage locations of the memory hosts; populatinga data array to have a size corresponding to a number of the datastripes in the file, the data array configured to receive data stripereplications of the data stripes in the file; instructing the memoryhosts to allocate storage of each of the data stripe replications acrossmultiple destination storage locations of the memory hosts; and migrateeach of the data stripe replications from the source storage locationsto the destination storage locations of the memory hosts; and committingeach of the data stripe replications to the data array.
 12. Thedistributed storage system of claim 11, wherein the data processinghardware further manages files stored across the memory hosts bydetermining a state of each data stripe replication, the stateindicating whether a respective data stripe replication is available forallocation to a respective destination storage location.
 13. Thedistributed storage system of claim 11, wherein the data processinghardware further manages files stored across the memory hosts by:receiving a memory access request for the file; determining that one ormore data stripe replications of the file are in a migrating state to arespective destination storage location of the memory hosts; and denyingthe memory access request for the file.
 14. The distributed storagesystem of claim 11, wherein instructing the memory hosts furthercomprises instructing the memory hosts to read the data stripes storedacross multiple source storage locations of the distributed storagesystem.
 15. The distributed storage system of claim 11, wherein the dataprocessing hardware further manages files stored across the memory hostsby accessing a file map mapping files to file descriptors to return afile descriptor corresponding to the data array in response to the copyrequest, the file descriptor configured to map the data stripereplications of the data stripes on respective memory hosts of thedistributed storage system.
 16. The distributed storage system of claim11, wherein the data processing hardware further manages files storedacross the memory hosts by providing at least one of a file stateattribute providing a state of the file, a data chunks attributeproviding a number of data stripe replications per stripe, a stripelength attribute providing a number of bytes per stripe, or a sub-stripelength attribute providing a number of bytes per sub-stripe.
 17. Thedistributed storage system of claim 11, further comprising a networkproviding communication between the memory hosts and the client, eachmemory host comprising a network interface controller in communicationwith its memory.
 18. The distributed storage system of claim 17, whereinthe network interface controller services remote direct memory accessrequests.
 19. The distributed storage system of claim 17, wherein thenetwork comprises at least one of an InfiniBand network or an Ethernetnetwork.
 20. The distributed storage system of claim 11, wherein thememory comprises at least one of dynamic random access memory and phasechange memory.